Old meets new to measure sea ice volume
Researchers add to their records using historic ships' logs.
Modern satellite technology lets scientists keep an eye on dwindling sea ice coverage in the Arctic Ocean, but that only takes them back 40 years.
To add a little more historical context, researchers from the University of Washington (UW) has stretched things back to 1901 using computer simulations and historic observations, some written by hand aboard precursors to today's US Coast Guard ships.
In what is known as the Old Weather project, they have worked with the US National Oceanic and Atmospheric Administration, the National Archives and citizen scientists to transcribe weather entries in digitised ships' logbooks.
"This extends the record of sea ice thickness variability from 40 years to 110 years, which allows us to put more recent variability and ice loss in perspective," says Axel Schweiger, first author of a paper published in the Journal of Climate.
The news isn’t great, however. The current rate of loss is “unprecedented” in that 110-year record.
In the early twentieth century, the few direct observations of sea ice were made by Revenue cutters, the precursor to the Coast Guard, and navy ships that have cruised through the Arctic each year since 1879.
"In the logbooks, officers always describe the operating conditions that they were in, providing hourly observations of the sea ice at that time and place," says co-author Kevin Wood.
If the ship was in open water, the logbook might read "steaming full ahead" or "underway". When it encountered ice, officers might write "steering various courses and speeds" meaning the ship was sailing through a field of ice floes. When they found themselves trapped in the ice pack, the log might read "beset".
While the historic sea ice observations have not yet been incorporated directly into UW’s Pan-Arctic Ice Ocean and Modelling System (PIOMAS), spot checks between the model and the early observations confirm the validity of the tool.
"This is independent verification that the model is doing the right thing," Schweiger said.
PIOMAS provides a daily reconstruction of what's happening to the total volume of sea ice across the Arctic Ocean. It combines weather records and satellite images of ice coverage to compute ice volume.
Schweiger says the new, longer record provides more context for big storms or other unusual events and a new way to study the Arctic Ocean sea ice system.
"The observations that we have for sea ice thickness and variability are so limited," he says. "I think people will start analysing this record. There's a host of questions that people can ask to help understand Arctic sea ice and predict is future."